1. Field of the Invention and Contract Statement
The invention relates to a method for thermally processing alumina-enriched spinel single crystals to obtain desired physical characteristics.
2. Discussion of Background
The physical characteristics--hardness, clarity, flexural strength, toughness--of magnesium aluminum spinels change significantly during thermal processing. These characteristics change at different rates and to different extents depending on the length of time and the temperature at which the thermal processing takes place.
By melting spinel compositions and then adding excess alumina, a single phase, supersaturated, solid solution of aluminum-enriched magnesium spinel can be formed ranging in composition from MgO:1 Al.sub.2 O.sub.3 to MgO: 3.5 Al.sub.2 O.sub.3. The single phase can be retained by rapid quenching to a room temperature where decomposition occurs. Subsequent thermal processing may result in precipitation of second phases depending on the processing temperature and time.
During thermal processing, as second phase crystals precipitate in the spinel matrix, matrix properties are transformed from those of the single phase material. For example, these second phase precipitates improve the hardness of the crystal by a mechanism known as age-hardening or transformation toughening. For high temperature structural and high strength optical applications, phase transformation-toughened crystals have great potential.
However, some of the physical characteristics of the spinel matrix do not improve with age-hardening; some change at different rates if age-hardening takes place at higher temperatures. The paths to desired physical characteristics wrought by age-hardening of alumina-enriched spinel crystals have not been obvious to previous researchers.
Phase transformed MgO:3.5Al.sub.2 O.sub.3 and its age-hardened properties have been a source of interest for many years. During World War II age-hardened MgO:3.5Al.sub.2 O.sub.3 was studied as a replacement for sapphire jewel bearings in instrumentation. In the 1960's, MgO:3.3Al.sub.2 O.sub.3 was considered as a substrate for silicon-integrated electronic devices. More recently, MgO:3.1Al.sub.2 O.sub.3 spinel has been tested for use as infrared, guided missile dome material and as electrical insulating material. The physical characteristics required of spinel crystals for these three applications are different.
French patent No. 1,136,953, issued Jan. 7, 1957 discloses e process for hardening magnesium spinels discovered by Forestier and Mengin. Three examples of age-hardened spinel crystals are described in the patent specification but the times end temperatures governing the age-hardening process are not definitively disclosed. The trend indicated by the three examples cited is that shorter times and higher age-hardening temperatures produce harder crystals.
In U.S. Pat. No. 3,990,902, Nishizawa discloses the substitution of magnesium titanate for a portion of magnesium aluminate to accommodate a layer of spinel epitaxially deposited as a substrate on a layer of silicon semiconductor material. The substitution avoids the mismatch that would otherwise occur between the spinel crystal lattice structure and that of silicon.
Others have examined MgO:x Al.sub.2 O.sub.3 (where x=3.1 to 3.5) but none has identified the thermal processing paths by which secondary phase transformations take place in alumina enriched spinel single crystals during thermal processing. Once these paths are known, the production of aluminn-enriched spinels with the desired physical characteristics for a wide variety of applications can be controlled and optimized.